Voice synthesis on ISR

This is a first attempt using 9 channels on OPLL chip
OPLL player

Thanks to wouter for his support.
The source code is here

I had some improvements, but not as much as I would have expected.
The main result is that now the channels with minimum volume are muted.
Here the new OPLL encoder and player

@wouter
The new OPLL encoder now has correct volumes and voice now sounds much better.
Never the less, looking at the sound wave in time we see something unexpected.
It seems like that the produced signal is modulated by a tone at 500Hz of frequency: one can see clearly a zero crossing each 0,002 seconds.

I'm using your settings as I'm not able to fiddle with OPLL registers.
Could it be due to the way the instrument is set ? And in case, can it be fixed somehow?

mod={AM=0 PM=0 EG=0 KR=0 ML=01 KL=0 TL=63 WF=0 FB=0 AR=15 DR=15 SL=15 RR=15}
car={AM=0 PM=0 EG=1 KR=0 ML=01 KL=0 WF=0 AR=15 DR=00 SL=00 RR=15}
Registers R#00-R#07 = { 01 21 3F 00 FF F0 FF 0F }

Just checked your opll.asm source code. The problem might be that you're writing the OPLL registers too fast. There should be at least 84 cycles (@3.57MHz) between 2 writes to IO port 0x7D (and 12 cycles between 2 writes to 0x7C). If I counted correctly your code should be fine in Z80 mode, but in R800 mode I think you're too fast.
OpenMSX doesn't emulate this too fast access yet.

Hmm...Sorry it was a false alarm.
We cannot reproduce the picture you see, it has been generated using bluemsx, but now when using open msx anything seems normal